# Calculate emf and internal resistance with two circuits with the same cell

## Homework Statement

A cell is connected to a 10.0 Ohm resistor. The pd across the 10.0 Ohm resistor is 10.43 V.
A 4.00 Ohm resistor is connected in parallel with the 10.0 Ohm resistor. The pd across the 10.0 Ohm resistor falls to 7.87 V.
Calculate the emf and the internal resistance of the cell.

E = V + Ir
V = IR

## The Attempt at a Solution

I tried to work out the total resistance in circuit 2. I got it as 2.86 Ohms. I then tried to work out the current in this circuit and got it as 2.75A. Using the information of volts, current and resistance I used Ohm's Law to try and work out the value of the internal resistor. I got it as 2.85 Ohms. However, when i used this in circuit 1 and tried to work out Emf, the two circuits did not match. Help?

a better way will be to use Kirchhoff's laws

you can find the current in 2 circuits using ohm's law and the potential drop across 2 resistor configuration.
then apply Kirchhoff loop law law and insert value of i in 2 circuits ... you'll get 2 eqn in 2 variables ... which you can solve

I'm doing AS physics and I haven't heard of Kirchhoff's law before.
Which equation would I use?

Kirchhoff laws says that net potential drop/gain in any closed circuit is 0

for this ... give any loop a sense of rotation (clockwise or anticlockwise) and take potential drop as negative and potential gain as positive or vice verse

then list all potential drop and potential gain and equate them to 0

check this:
http://www.globalshiksha.com/content/examples-of-kirchhoffs-question [Broken]
http://en.wikipedia.org/wiki/Kirchhoff's_circuit_laws

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